A Key Stone (Japanese: キーストーンKey Stone) is a Key Item used to allow a Pokémon holding a Mega Stone to Mega Evolve. When the Key Stone is activated, the Pokémon's corresponding Mega Stone reacts to it, starting the process.

Trainers typically embed their Key Stone in a personal accessory, such as a Mega Ring, Mega Bracelet, Z-Ring, or Z-Power Ring.

In the core series games

Player character

Pokémon X and Y

In Pokémon X and Y, the player characters hold their Key Stones in a Mega Ring (Japanese: メガリングMega Ring), a large black bracelet that Calem and Serena wear on their left wrist.

The possession of one Mega Ring is offered to the player and each of his or her friends, who have to pick the most deserving out of their group. The selection is quickly whittled down to two people: the player and Serena or Calem (depending on the gender of the player). Upon defeating their rival, the player must obtain the Rumble Badge at the Shalour Gym and then go to the top of the Tower of Mastery to receive the ring from Korrina.

After the player has beaten their rival in Kiloude City, Professor Sycamore will upgrade their Mega Ring at the Anistar Sundial by having the player touch the sundial. Once upgraded, the player can then find hidden Mega Stones around Kalos from 8:00 PM to 9:00 PM.

The player character also wears a special Mega Bracelet during Pokémon Contest Spectaculars, Brendan's being a Mega Cuff (Japanese: メガブレスMega Bracelet), a black band with a circle of large, connected red diamonds affixed to it, with the Key Stone embedded in the band, while May's is a Mega Charm (Japanese: メガチャームMega Charm), a chain of cream bows with the Key Stone embedded into a pink heart charm.

Secret Base Trainer avatars of player characters that have a Mega Bracelet will use it in battle if possible.

In the same episode, Astrid was shown wearing her Key Stone as an earring on her right ear, while Remo had his Key Stone embedded into the tip of the umbrella that he carries with him. Siebold, a member of the Kalos Elite Four, was seen wearing his Key Stone in his necktie. Astrid's Key Stone made a brief reappearance during the opening credits of Diancie and the Cocoon of Destruction.

In Mega Revelations!, Korrina was shown wearing her Key Stone in her Mega Glove, having received it from her grandfather. However, her Lucario lacked the Lucarionite, meaning that she wasn't able to use it yet. After obtaining a Lucarionite in The Cave of Trials!, she was soon able to use her Key Stone in order to Mega Evolve her Lucario.

Another Key Stone appeared in The Aura Storm!, under the ownership of Gurkinn, who uses it to Mega Evolve his own Lucario. Similar to Korrina, he wears it in a glove on his left hand.

In the same episode, a giant Key Stone was discovered by Steven and Alain. While Lysandre's scientists were inspecting it, Rayquaza appeared and used the Key Stone's power to Mega Evolve into Mega Rayquaza. Soon after, it disappeared along with the chaos created by Rayquaza's Draco Meteor. The stone reappeared in Mega Evolution Special III, along with Mega Rayquaza. It was eventually taken away by Lysandre's submarine to be studied at the Lysandre Labs.

In Garchomp's Mega Bond!, it was revealed that Professor Sycamore had found a Key Stone along with a piece of Garchompite. In the same episode, it was revealed that Blaziken Mask keeps his Key Stone embedded in his belt buckle.

Multiple Key Stones appeared in Mega Evolution Special IV. A Trainer with a MegaVenusaur has his Key Stone inside of a pocket watch, a MegaAlakazam Trainer wears his Key Stone on his turban, Malva carries her Key Stone in a necklace, a MegaBanette Trainer has his embedded on his Banette puppet, and Lysandre has his Key Stone embedded in a Mega Ring he wears on his left hand.

In Pokémon Generations

In The Vision, Brendan and Maxie were seen wearing their Mega Bracelet and Mega Glasses, respectively. Furthermore, Archie could be seen wearing his Mega Anchor in The Cavern. None of them was seen using their Key Stones, however.

Those taught by Gurkinn, the Mega Evolution guru, tend to use Mega Gloves. This includes Korrina, three unnamed students, and Gurkinn himself. Gurkinn stated that he gave Key Stones to 16 people through a Mega Evolution successor ceremony, with X being the sole exception. He later gave his Key Stone to Y, thus making her a successor as well.

At Lysandre Café in Lumiose City, Diantha shields Y and her friends from Lysandre's Pokémon by using her Mega Charm to Mega Evolve her Gardevoir into Mega Gardevoir.

Lysandre later obtains a Mega Ring from Malva outfitted with a Key Stone stolen from Korrina.

At the Lost Hotel, Y is made a Mega Evolution successor by Gurkinn. She is given a Mega Ring outfitted with the Key Stone from Gurkinn's Mega Glove. With it, she can Mega Evolve her Solsol into Mega Absol.

Blue is later revealed to be a successor as well, keeping his Key Stone on a ring. When not in use, Blue wears the ring around his neck as a pendant. With it, he can Mega Evolve his Charizard into Mega Charizard Y.

Blaine later appears at the Pokémon Village, revealed to have reunited with Mewtwo. He had gained a Key Stone sometime prior and placed it in his cane, using it to Mega Evolve Mewtwo into either Mega Mewtwo Y or Mega Mewtwo X.

Many members of the Draconid people possess Key Stones, such as Zinnia, Renza, Jinga, Tomatoma, and the Meteor Village Elder. While Zinnia keeps her Key Stone in her Mega Anklet, Renza, Jinga, and Tomatoma keep theirs in arm bands, similar to Zinnia's anklet, while the Elder keeps hers in a walking cane. When battling Ruby and Sapphire, Zinnia wears a second Mega Anklet that originally belonged to Aster.

In Omega Alpha Adventure 6, Steven Mega Evolved his Metagross into Mega Metagross with his Mega Stickpin. In PAORAS20, he reveals that he is a Mega Evolution successor, having obtained the Key Stone from Gurkinn. He later gives Zinnia a jar of Key Stones belonging to Aster that were taken from the Embedded Tower.

When battling against Maxie and Archie, Red and Green use rings with patterns similar to the Mega Bracelets to Mega Evolve Saur and Blasty respectively to battle them. Though their rings resemble the one owned by Blue, it is unknown where they received them.

Trivia

In Generation VI, before battle with a non-player character that can use Mega Evolution, the Key Stone accessory is often shown off. For Trainers that use VS sprites (such as Korrina and Diantha), often the Key Stone is clearly visible in the zoomed-in VS sprite. For Trainers that use 3D models (such as Lysandre and Zinnia), the camera often starts by focusing on the Key Stone before zooming out to show the full body. Some Trainers, such as Calem/Serena and Brendan/May, have the Key Stone accessory visible on their 3D model, but it is not focused on. However, other Trainers, such as Courtney, Matt and the HoennElite Four, do not have visible Key Stones at all. Secret Base Trainers have no visible indication of possessing a Key Stone.